Engineering Guide June 28, 2026 12 min read

Valve Pressure Class Ratings: ANSI 150 to 2500 Explained

Every industrial valve has a pressure class rating that defines the maximum allowable working pressure at a given temperature. Understanding pressure classes (ANSI/ASME 150, 300, 600, 900, 1500, 2500) is fundamental to valve selection and ensures safe operation. This guide explains pressure-temperature relationships, material effects, and how to select the right class.

What is a Valve Pressure Class?

A pressure class (also called pressure rating or class designation) is a standardized number that defines the pressure-temperature capability of a valve. The class number itself is not a direct pressure value in any unit — it is an index that maps to specific pressure-temperature tables in ASME B16.34 (for valves) and ASME B16.5 (for flanges).

For example, a Class 150 WCB valve is rated for 285 psig (19.6 barg) at -29°C to 38°C, but this rating decreases at higher temperatures because the material loses strength.

Standard Pressure Classes per ASME B16.34

150

285 psig @ 100°F

19.6 bar @ 38°C

300

740 psig @ 100°F

51.0 bar @ 38°C

600

1480 psig @ 100°F

102.1 bar @ 38°C

900

2220 psig @ 100°F

153.1 bar @ 38°C

1500

3705 psig @ 100°F

255.3 bar @ 38°C

2500

6170 psig @ 100°F

425.4 bar @ 38°C

Pressure-Temperature Tables by Material

The allowable working pressure decreases as temperature increases because material strength decreases at elevated temperatures. Different materials have different P-T curves. Here are the key values for the most common valve body materials:

WCB Carbon Steel (ASTM A216 Group 1.1)

Temp	Class 150	Class 300	Class 600	Class 900	Class 1500
-29 to 38°C	19.6 bar	51.1 bar	102.1 bar	153.2 bar	255.3 bar
100°C	18.3 bar	47.6 bar	95.2 bar	142.8 bar	237.9 bar
200°C	15.8 bar	41.2 bar	82.4 bar	123.6 bar	206.0 bar
300°C	13.8 bar	35.8 bar	71.6 bar	107.4 bar	179.0 bar
425°C (max)	9.3 bar	24.2 bar	48.3 bar	72.5 bar	120.9 bar

Key takeaway: A Class 150 WCB valve rated at 19.6 bar at ambient temperature is only good for 9.3 bar at 425°C — a 53% reduction. Always check the full P-T table, not just the ambient rating.

How to Select the Right Pressure Class

Determine design pressure: From process data (P&ID, line list, or process datasheet)
Determine design temperature: Maximum temperature the valve will see (not normal operating temperature)
Identify body material: WCB, SS316, WC6, WC9, etc. from the piping material specification
Look up P-T table: Find the allowable pressure at design temperature in ASME B16.34 Table 2
Select class: Choose the lowest class where the allowable pressure exceeds the design pressure
Verify with piping spec: The piping class table may require a higher class than the minimum calculated

Common Pressure Class Applications

Class 150: Water treatment, HVAC, low-pressure process, utility piping (up to 19.6 bar)
Class 300: Medium-pressure process, steam (up to 51 bar), standard refinery utility
Class 600: High-pressure process, main oil/gas pipelines, high-temperature steam
Class 900: High-pressure gas, HP steam, hydraulic systems
Class 1500: Very high pressure, wellhead service, HP gas injection
Class 2500: Extreme pressure, HP/HT wellheads, specialized petrochemical reactors

Frequently Asked Questions

Why does the pressure rating decrease at higher temperatures?

All metals lose yield strength and tensile strength as temperature increases. The ASME B16.34 pressure-temperature tables account for this by reducing the allowable working pressure in proportion to the decrease in material strength. For example, WCB carbon steel has a yield strength of ~248 MPa at room temperature but only ~159 MPa at 425°C, which is why the Class 150 rating drops from 19.6 bar to 9.3 bar.

What is the difference between PN and ANSI Class ratings?

PN (Pression Nominale) is the European pressure designation per EN standards, while ANSI Class is the US designation per ASME standards. They are not directly interchangeable: PN16 is roughly equivalent to Class 150, PN40 to Class 300, and PN64 to Class 600 — but the pressure-temperature curves differ because they reference different material standards. For projects using ASME piping codes, always specify ANSI Class. For EN 1092-1 flanges, specify PN.

Can I use a Class 300 valve in a Class 150 system?

Yes, a higher-class valve can always be used in a lower-class system — it simply has more pressure capacity than needed. However, the flanges must match: you cannot bolt a Class 300 flange to a Class 150 flange because the bolt patterns differ for sizes above 2". Using a higher class adds cost and weight, so it is only justified for spare parts standardization or future uprating.

What does CWP (Cold Working Pressure) mean on a valve?

CWP is the maximum non-shock working pressure at ambient temperature (-29°C to 38°C). For threaded and socket weld valves per API 602, the CWP rating is often stamped directly on the body — e.g., "2000 CWP" means 2000 psi at ambient temperature. CWP values may exceed the calculated ASME B16.34 Class rating because forged bodies have higher strength than the minimum specified by the Class tables.

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